Method for improving strength and retention, and paper product

ABSTRACT

A method for improving strength and retention in the manufacture of paper includes providing a composition containing microfibrillated cellulose in a fiber suspension, and from 0.1 to 10 w-% of microfibrillated cellulose by mass of the fiber suspension is added to improve the strength and retention of the product to be formed. A corresponding paper product is also provided.

This application is a Continuation-in-Part of U.S. Ser. No. 13/980,088,filed 17 Jul. 2013, which is a National Stage Application ofPCT/FI2012/050045, filed 19 Jan. 2012, which claims benefit of Ser. No.20/115,054, filed 20 Jan. 2011 in Finland and which applications areincorporated herein by reference. To the extent appropriate, a claim ofpriority is made to each of the above disclosed applications.

FIELD

The invention relates to a method of making paper products, and to paperproducts with improved strength and/or retention.

BACKGROUND

Known from the prior art are different methods for manufacturing paperpulp and paper products. In addition, it is known from the prior art toimprove the properties of paper products by different filler and coatingmaterials, e.g. pigments, in connection with papermaking. It is knownthat the aim in papermaking is to provide the best properties possiblefor the paper product.

Retention and strength problems are known form papermaking. Thestrength, particularly dry strength, of the product to be formed is animportant property of the product which is typically tried to beimproved. In addition, the retention of small particles, such as fillersand fines, is important in papermaking. Retention means the ratio of thefiber and filler material remaining on the wire to the material that hasbeen fed, i.e. it means the ability of the wire to retain fiber pulp.Know are different retention agents for improving retention. Theretention agents provide suitable fixation of the fibers, fillers andother chemicals of the fiber pulp to the web. Known retention agentsinclude e.g. polyacrylamides and combined retention agents, such ascombinations of anionic and cationic retention agents. In addition, itis known to use a combination of polyacrylamide and microparticles as aretention agent.

On the other hand, it is known from the prior art to manufacturemicrofibrillated cellulose and use it in the manufacture of paper pulpand paper products. In studies on microfibrillated cellulose, it hasbeen found that microfibrillated cellulose improves the strength ofpaper, i.a. Microfibrillated cellulose has a large specific surface areaand has thus more bonding area relative to material weight.

OBJECTIVE

The objective of the invention is to disclose a new type of a method forimproving strength as well as retention in papermaking, and acorresponding paper product.

SUMMARY

The method and the corresponding paper product according to theinvention are characterized by what has been presented in the claims.

The invention is based on a method for improving strength and retentionin papermaking. According to the invention, a composition containingmicrofibrillated cellulose is provided in a fiber suspension, preferablypaper pulp, and from 0.1 to 10 w-% of microfibrillated cellulose by massof the fiber suspension is added to improve the strength, e.g. drystrength, tensile strength of dry paper, internal bond strength and/orinitial wet strength, and retention of the product to be formed.

Fiber suspension in this context means any suspension of fiber-basedpulp containing a fiber-based composition that may be formed from anyplant-based raw material, e.g. wood-based raw material, such as hardwoodraw material or softwood raw material, or other plant raw materialcontaining fibers, such as cellulose fibers. The fiber suspension may befiber-based pulp formed by a chemical method wherein the fibers havebeen separated from each other and most of the lignin has been removedby chemicals using a chemical method that may be e.g. a sulfate process,sulfite process, soda process, a process based on organic solvents orother chemical treatment method known per se in the art. Alternatively,the fiber suspension may be fiber-based pulp formed by a mechanicalmethod, for example TMP, PGW, CTMP or the like.

In one embodiment, the composition containing microfibrillated cellulosemay be in the form of a dispersion, e.g. in a gel-type or gelatinousform or in the form of a diluted dispersion, or in the form of asuspension, e.g. aqueous suspension. Preferably, the compositioncontaining microfibrillated cellulose is in the form of an aqueoussuspension. The composition may contain from more than 0% to less than100 w-% of microfibrillated cellulose. In one embodiment, thecomposition may consist mainly of microfibrillated cellulose. Inaddition to microfibrillated cellulose, the composition may containother suitable components, e.g. fibers that may be formed from anyplant-based raw material, and/or different additives and/or fillers.

Microfibrillated cellulose in this context means cellulose consisting ofmicrofibrils, i.e. a set of isolated cellulose microfibrils and/ormicrofibril bundles derived from a cellulose raw material. Cellulosefibers contain microfibrils that are strand-like structural componentsof the cellulose fibers. The cellulose fiber is provided fibrous byfibrillating. The aspect ratio of microfibrils is typically high; thelength of individual microfibrils may be more than one micrometer andthe number-average diameter is typically less than 20 nm. The diameterof microfibril bundles may be larger but generally less than 1 μm. Thesmallest microfibrils are similar to the so-called elementary fibrils,the diameter of which is typically from 2 to 4 nm. The dimensions andstructures of microfibrils and microfibril bundles depend on the rawmaterial and production method.

Microfibrillated cellulose may have been formed from any plant-based rawmaterial, e.g. wood-based raw material, such as hardwood raw material orsoftwood raw material, or other plant-based raw material containingcellulose. Plant-based raw materials may include e.g. agriculturalwaste, grasses, straw, bark, caryopses, peels, flowers, vegetables,cotton, maize, wheat, oat, rye, barley, rice, flax, hemp, abaca, sisal,kenaf, jute, ramie, bagasse, bamboo or reed or their differentcombinations.

Microfibrillated cellulose may also contain hemicellulose, lignin and/orextractives, the amount of which depends on the raw material used.Microfibrillated cellulose is isolated from the above-described rawmaterial containing cellulose by an apparatus suitable for the purpose,e.g. a grinder, pulverizer, homogenizer, fluidizer, micro- ormacrofluidizer, cryo-crushing and/or ultrasonic disintegrator.Microfibrillated cellulose may also be obtained directly by afermentation process using microorganisms e.g. from the generaAcetobacter, Agrobacterium, Rhizobium, Pseudomonas or Alcailgenes, mostpreferably from the genera Acetobacter and most preferably of all fromthe species Acetobacter xylinum or Acetobacter pasteurianus. Rawmaterials of microfibrillated cellulose may also include for example thetunicates (Latin: tunicata) and organisms belonging to thechromalveolate groups (Latin: chromalveolata), e.g. the water molds(Latin: oomycete), that produce cellulose.

In one embodiment, microfibrillated cellulose may be any chemically orphysically modified derivative of cellulose or microfibril bundlesconsisting of microfibrils. The chemical modification may be based one.g. a carboxymethylation, oxidation, esterification and etherificationreaction of the cellulose molecules. The modification may also becarried out by physical adsorption of anionic, cationic or non-ionicagents or their combinations to the surface of cellulose. Themodification may be performed before, during or after the manufacture ofmicrofibrillated cellulose.

Microfibrillated cellulose may be formed from a cellulose-based rawmaterial by any manner known per se in the art. In one embodiment,microfibrillated cellulose is formed from a dried and/or concentratedcellulose raw material by fibrillating. In one embodiment, the celluloseraw material has been concentrated. In one embodiment, the cellulose rawmaterial has been dried. In one embodiment, the cellulose raw materialhas been dried and concentrated. In one embodiment, the cellulose rawmaterial has been chemically pretreated to disintegrate more easily,i.e. labilized, in which case microfibrillated cellulose is formed fromthe chemically labilized cellulose raw material. For example, a N-oxyl(e.g. 2,2,6,6-tetramethyl-1-piperidine N-oxide)-mediated oxidationreaction provides a very labile cellulose raw material that isexceptionally easily disintegrated into microfibrillated cellulose. Sucha chemical pretreatment is described for example in patent applicationsWO 09/084566 and JP 20070340371.

The fibrils of microfibrillated cellulose are fibers that are very longrelative to the diameter. Microfibrillated cellulose has a largespecific surface area. Therefore, microfibrillated cellulose is able toform multiple bonds and bind many particles. In addition,microfibrillated cellulose has good strength properties.

In one embodiment, microfibrillated cellulose is at least partially ormainly nanocellulose. Nanocellulose consists at least mainly ofnano-size class fibrils, the diameter of which is less than 100 nm butthe length of which may also be in the μm-size class or below.Alternatively, microfibrillated cellulose may also be referred to asnanofibrillated cellulose, nanofibril cellulose, nanofibers ofcellulose, nanoscale fibrillated cellulose, microfibril cellulose ormicrofibrils of cellulose. Preferably, microfibrillated cellulose inthis context does not mean so-called cellulose nanowhiskers ormicrocrystalline cellulose (MCC).

In one embodiment of the invention, a composition containing cationicmicrofibrillated cellulose is added to the fiber suspension.

In one embodiment of the invention, a composition containing anionicmicrofibrillated cellulose is added to the fiber suspension.

In one embodiment of the invention, the composition contains a componentcontaining microfibrillated cellulose, and a filler, e.g. PCC.

In one embodiment of the invention, the composition contains a componentcontaining microfibrillated cellulose, and a fiber-based solid material,e.g. fines.

In one embodiment, the composition contains an additive, e.g. an AKDsizing agent, ASA sizing agent or corresponding additives.

In one embodiment of the invention, the component containingmicrofibrillated cellulose in the composition is anionic. In oneembodiment, the component containing microfibrillated cellulose isanionic and the filler is cationic.

In one embodiment of the invention, the component containingmicrofibrillated cellulose in the composition is cationic. In oneembodiment, the component containing microfibrillated cellulose iscationic and the filler is anionic.

In one embodiment of the invention, a composition containing anionicand/or cationic microfibrillated cellulose is added to the fibersuspension including a filler. In one embodiment, a compositioncontaining anionic microfibrillated cellulose is added to the fibersuspension including as a filler a cationic filler, e.g. PCC.

In one embodiment of the invention, a composition containing anionicand/or cationic microfibrillated cellulose is added to the fibersuspension including fines, in one embodiment fiber-based fines.

In one embodiment, a composition containing anionic and/or cationicmicrofibrillated cellulose is added to the fiber suspension including anadditive.

In one embodiment, a composition containing anionic and/or cationicmicrofibrillated cellulose is added to the fiber suspension including afiller, fines and/or an additive.

In one embodiment of the invention, a cationic polyelectrolyte is addedto the composition containing microfibrillated cellulose.

In one embodiment of the invention, an anionic polyelectrolyte is addedto the composition containing microfibrillated cellulose.

In one embodiment of the invention, inorganic nano- and/ormicroparticles, e.g. SiO₂ particles, are added to the compositioncontaining microfibrillated cellulose. In one embodiment, inorganicnano- and/or microparticles are added to the composition containingcationic microfibrillated cellulose. In one embodiment, apolyelectrolyte and inorganic nano- and/or microparticles are added tothe composition containing microfibrillated cellulose.

In one embodiment of the invention, from 1 to 5 w-%, in one preferredembodiment from 1 to 3 w-%, of microfibrillated cellulose by mass of thefiber suspension is added to the fiber suspension.

In one embodiment of the invention, at least part of the retentionchemicals and/or strength chemicals is replaced by the compositioncontaining microfibrillated cellulose. In one embodiment, part of theconventional retention chemicals and/or strength chemicals is replacedby the composition containing microfibrillated cellulose. In oneembodiment, the conventional retention chemicals and/or strengthchemicals are entirely replaced by the composition containingmicrofibrillated cellulose. In one embodiment wherein the conventionalretention chemicals are entirely replaced, a composition containing bothcationic microfibrillated cellulose and anionic microfibrillatedcellulose is used. In one embodiment, one of the components, e.g. apolymer component or microparticle component, is replaced in a2-component retention arrangement. In one embodiment wherein a polymercomponent is replaced, a composition containing cationicmicrofibrillated cellulose is used. In one embodiment wherein amicroparticle component is replaced, a composition containing anionicmicrofibrillated cellulose is used. In one embodiment, at least onecomponent in a multicomponent retention arrangement is replaced.

In one embodiment of the invention, the method is used in themanufacture of a fiber suspension containing microfibrillated cellulose.In one embodiment of the invention, the method is used in themanufacture of paper pulp.

In one embodiment of the invention, the method is used in papermaking.The method according to the invention can be applied for use in themanufacture of different paper products wherein the paper product isformed from the fiber-based composition. A paper product in this contextmeans any fiber-based paper, board or fiber product or an equivalentproduct. The paper product may have been formed from chemical pulp,mechanical pulp, chemimechanical pulp, recycled pulp, fiber pulp and/orplant-based pulp. The paper product may contain suitable fillers andadditives as well as different surface treatment and coating agents.

In one embodiment of the invention, the method is used in themanufacture of a product containing microfibrillated cellulose, e.g. inthe manufacture of different compositions and mixtures, preferably inthe manufacture of precipitated compositions and mixtures, in themanufacture of different films, in the manufacture of differentcomposite products or in equivalent cases. In one embodiment, the methodis mainly used in the manufacture of a product containingmicrofibrillated cellulose, such as in the manufacture of a precipitatedmicrofibril cellulose suspension or in the manufacture of films formedfrom microfibrillated cellulose.

In addition, the invention is based on a corresponding paper productformed from the fiber-based composition. According to the invention, thepaper product contains microfibrillated cellulose such that acomposition containing microfibrillated cellulose has been added to afiber suspension, containing the fiber-based composition, in an amountof from 0.1 to 10 w-% by mass of the fiber suspension, and the paperproduct has an improved retention and strength.

The invention provides considerable advantages relative to the priorart.

Thanks to the invention, the retention and strength in a paper productcontaining microfibrillated cellulose can be improved. The retention ofthe filler or retention of the additive or retention of the entire fibersuspension can be influenced by the solution according to the invention.

Thanks to the invention, the quality of the paper product to be formedcan be improved and additionally the raw material and energyexpenditures can be reduced.

The method according to the invention is easily industrially applicable.

In addition, the invention provides for a new method of use formicrofibrillated cellulose.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graphical representation of drainage time as a function ofNFC dosage according to Example 3.

FIG. 1B is a graphical representation of retention as a function of NFCdosage according to Example 3.

FIG. 1C is a graphical representation of grammage as a function of NFCdosage according to Example 3.

FIG. 1D is a graphical representation of apparent bulk density as afunction of NFC dosage according to Example 3.

FIG. 1E is a graphical representation of tensile index as a function ofNFC dosage according to Example 3.

FIG. 1F is a graphical representation of bonding strength as a functionof NFC dosage according to Example 3.

FIG. 1G is a graphical representation of air permeance as a function ofNFC dosage according to Example 3.

FIG. 2A is a graphical representation of drainage time as a function ofNFC dosage according to Example 4.

FIG. 2B is a graphical representation of retention as a function of NFCdosage according to Example 4.

FIG. 2C is a graphical representation of grammage as a function of NFCdosage according to Example 4.

FIG. 2D is a graphical representation of apparent bulk density as afunction of NFC dosage according to Example 4.

FIG. 2E is a graphical representation of tensile index as a function ofNFC dosage according to Example 4.

FIG. 2F is a graphical representation of bonding strength as a functionof NFC dosage according to Example 4.

FIG. 2G is a graphical representation of air permeance as a function ofNFC dosage according to Example 4.

DETAILED DESCRIPTION

According to at least some embodiments, the method comprises adding acomposition comprising anionically modified nanofibrillated cellulose(“anionic NFC”) to a fiber suspension to produce a modified fibersuspension, and preparing a paper product from the modified fibersuspension. The anionic NFC can be added to the fiber suspension at aconcentration of about 0.1 to about 10 wt-% (on a dry weight basis),about 0.1 to about 5 wt-%, about 0.2 to about 2 wt-%, or about 0.4 toabout 1 wt-%. In some embodiments the anionic NFC is added at aconcentration of about 1 wt-%.

The method may further include adding native (chemically unmodified)nanofibrillated cellulose (“native NFC”), a starch addition (e.g.,cationic starch), retention aids (e.g., cationic polyacrylamide), andfillers to the fiber suspension or modified fiber suspension. Cationicstarch may be added at any suitable concentration, such as 0 to about 10wt-% (on a dry weight basis), about 0.1 to about 5 wt-%, or about 0.5 toabout 2 wt-%. In some embodiments the cationic starch is added at aconcentration of about 1 wt-%.

The fiber suspension may include a suspension of any fiber-based pulpformed from a plant-based raw material, e.g. wood-based raw material,such as hardwood raw material or softwood raw material, or other plantraw material containing fibers, such as cellulose fibers. The fibersuspension may comprise a chemical pulp or a mechanical pulp.

The additives and their amounts added to the fiber suspension to producea modified fiber suspension can be selected based on a desired endresult. For example, the additives and their amounts can be selected toincrease or reduce drainage time; to increase retention; to increase thetensile index; to increase boding strength; and/or to increase or reducepermeability of the resulting paper product.

The invention will be described in more detail by the accompanyingexamples.

EXAMPLE 1

The retention of a fiber suspension containing PCC was studied.Nanocellulose was added to the fiber suspension. The fiber suspensionwas the pulp to be used for the manufacture of a paper product.

Anionic nanocellulose was used to bind cationic particles, such asprecipitated calcium carbonate (PCC), in order to increase the retentionof fines in the fiber suspension. 3 w-% of anionic nanocellulose wasadded to the fiber suspension containing 20 w-% of precipitated calciumcarbonate (PCC). Sheets were formed from the fiber suspension. Theretention was determined for the obtained sheet to which nanocellulosehad been added. As a reference, the retention was also determined for asheet formed from a fiber suspension containing 20 w-% of precipitatedcalcium carbonate (PCC) but no nanocellulose. In addition, the wetstrengths were determined for the sheets.

It was found that the retention of the filler, i.e. PCC, could besignificantly improved by the solution according to the invention. Theretention was improved from 62% to 84%. In addition, it was found thatthe dry strength of the product was improved. It was discovered that theeffect was provided by virtue of the physical and chemical properties ofnanocellulose. Due to the wide specific surface area of nanocelluloseand high aspect ratio of the microfibrils, nanocellulose formed anetwork structure within the product composition already at very dilutedaqueous suspensions, which improved both strength and retention. It wasfound that anionic nanocellulose flocked cationic PCC, whereby it ismore effectively retained by the fibers.

In addition, the effect of the amount of addition of nanocellulose onthe retention was studied. It was found that as the amount ofnanocellulose increased from 1 w-% to 3 w-% in the fiber suspensionincluding 20 w-% of precipitated calcium carbonate, the retention ofprecipitated calcium carbonate increased from 75% to 82%. In addition,it was found that as the amount of nanocellulose increased from 3 w-% to6 w-%, the retention of precipitated calcium carbonate slightlyincreased further, yet not significantly.

EXAMPLE 2

The effect of addition of cationic nanocellulose on the dry strength ofa product was studied using the tensile index. 20, 30 and 45 mg/g ofcationic nanocellulose were added to fiber pulp 1 including a smallamount of fines (10 min. grinding) and to fiber pulp 2 including morefines (30 min. grinding). Sheets were formed from the fiber pulps andthe strengths were determined. Pine chemical pulp was used as the fiberpulp.

It was found that the strength of the sheet formed from fiber pulp 1 waslower than the strength of the product formed from a referencecomposition including 10 mg/g of cationic starch and 20, 30 and 45 mg/gof anionic nanocellulose. In addition, it was found that the strength ofthe sheet formed from fiber pulp 2 was clearly better that the strengthof the sheet formed from fiber pulp 1. Thus, the effect of cationicnanocellulose on the strength was clearly higher, which was due to thefact that cationic nanocellulose retained the fines, whereby thestrength of the sheet was improved. On this basis, starch can bereplaced by nanocellulose for a strengthening purpose.

EXAMPLE 3

The effect of microfibrillated cellulose, i.e., nanofibrillatedcellulose (NFC), on the properties of the resulting paper product wastested. Nanofibrillated cellulose was added at 0.2 to 1.0% by weight (2to 10 kg/t). All amounts are given on a dry-weight basis. The effect ofanionically treated nanofibrillated cellulose was compared with nativechemically unmodified nanofibrillated cellulose.

Canadian Standard Freeness (CSF) level of pulp describes the degree ofbeating/refining of the pulp and is a measure of drainage resistance.The unit of CSF is mL, and higher values indicate slower filtration andthus higher degree of beating/refining. The term “beating” is used withregard to chemical pulp, and the term “refining” is used with regard tomechanical pulp.

Raw Materials:

-   -   Chemical pulp: Kaukas Pinus produced by Kaukas pulp mill, beaten        to Canadian Standard Freeness (CSF) level of 605 mL.    -   Mechanical pulp: Pressure ground wood (PGW) from Kaukas paper        mill, CSF level 67 mL.    -   Modified nanofibrillated cellulose (“Anionic NFC”): UPM        Biofibrils AS83, lot 11851, supplied as a gel with solids        content 2.52% by weight, available from UPM Kymmene Corp. in        Helsinki, Finland. The Anionic NFC was modified to result in a        surface charge that was more anionic than unmodified NFC.    -   Native chemically unmodified nanofibrillated cellulose (“Native        NFC”): UPM Biofibrils NS 11246, supplied as a gel with solids        content 1.5% by weight, available from UPM Kymmene Corp. in        Helsinki, Finland    -   Reference (“REF”): without nanofibrillated cellulose    -   Retention aid: cationic polyacrylamide (FENNOPOL 3400R,        available from Kemira Oyj in Helsinki, Finland)    -   Water (osmotically purified)    -   No filler was used

Method:

1. Dilution and activation of nanofibrillated cellulose:

-   -   A. Anionic NFC and Native NFC were each diluted to 0.3% solids        with water.    -   B. The diluted NFC compositions were mixed with an immersion        mixer (BAMIX), carried out in three 10 s mixing periods.

2. Preparation of samples:

-   -   A. Mechanical pulp and chemical pulp were mixed at a ratio of        3:1 (mechanical pulp to chemical pulp).    -   B. To each sample, either Anionic NFC or Native NFC was added        according to TABLE 1.    -   C. The samples were mixed for 5 minutes.    -   D. Retention aid was added to the samples at 50 g/t immediately        prior to sheet making.

3. Sheets were prepared from each sample using a circulation sheet mold.

4. Sheets were dried using gloss plates and air conditioned beforemeasurements.

TABLE 1 NFC Dosage. Sample NFC Type NFC Dose (kg/t) 1 (REF) None 0 2Anionic NFC 2 3 Anionic NFC 6 4 Anionic NFC 10 5 Native NFC 2 6 NativeNFC 6 7 Native NFC 10

The sheets were evaluated for various properties, including drainagetime, grammage (weight per area), bulking thickness, bulk density,tensile strength, stretch at break, tensile energy absorption (TEA), TEAindex, tensile stiffness, tensile stiffness index, breaking length,bonding strength (Scott Bond), air permeability (measured by theBendtsen method), and retention. Retention was determined by measuringweight of material going in to each sheet mold vs. the weight of sheet.Results are shown in TABLE 2 and FIGS. 1A-1G.

TABLE 2 Results. SAMPLE 1 (REF) 2 3 4 5 6 7 NFC dose (%) 0 0.2 0.6 1 0.20.6 1 NFC type None Anionic Anionic Anionic Native Native NativeDrainage time (s) 17.8 19.5 19.1 20.1 18.8 20.1 20.9 Grammage (g/m²)60.9 61.0 61.1 60.8 60.7 60.8 60.4 Bulking thickness (μm) 115 116 114113 116 115 114 Apparent bulk density 532 527 537 539 523 527 528(kg/m³) Tensile strength (kN/m) 2.62 2.72 2.77 2.79 2.69 2.75 2.77Tensile index (Nm/g) 43.1 44.6 45.3 45.9 44.3 45.2 45.9 Stretch at break(%) 2.3 2.4 2.2 2.3 2.4 2.3 2.4 TEA (J/m²) 41 45 43 45 44 44 46 TEAindex (J/kg) 670 740 700 744 730 720 762 Tensile stiffness (kN/m) 302308 315 308 307 308 311 Tensile stiffness index 5.0 5.1 5.2 5.1 5.1 5.15.2 (MNm/kg) Breaking length (m) 4395 4549 4619 4680 4517 4609 4682Bonding strength, SB 298 301 311 322 298 308 308 Low (J/m²) Airpermeability, 131 134 125 124 150 128 119 Bendtsen (ml/min) Retention(%) 96.1 95.4 95.6 96.8 95.0 96.3 96.5

It was observed that addition of NFC (either anionic or native)increased the drainage time. In case of higher dosage amounts, anionicNFC has had slightly lower drainage time than native NFC. Drainage timeas a function of NFC dosage is shown in FIG. 1A.

Only marginal differences in absolute retention values were seen.Retention with low NFC dosage was slightly below reference, and slightlyabove with high NFC dosage. Retention as a function of NFC dosage isshown in FIG. 1B.

The sheets containing native NFC had generally slightly lower grammagethan sheets containing anionic NFC. However, the differences in grammagewere not great. Grammage as a function of NFC dosage is shown in FIG.1C.

It was observed that density did not vary greatly from one sample to thenext. Sheets containing anionic NFC had slightly higher density thannative NFC containing sheets. The reference had higher density thansheets having the lowest NFC content and the samples containing nativeNFC. Bulk density as a function of NFC dosage is shown in FIG. 1D.

It was observed that NFC increased tensile above reference at all dosagelevels. Tensile increased as a function of NFC dosage amount. Tensileindex as a function of NFC dosage is shown in FIG. 1E.

NFC addition increased Scott Bond level of the samples. Anionic NFC hada higher effect on Scott Bond than native NFC. Bonding strength as afunction of NFC dosage is shown in FIG. 1F.

NFC addition was found to decreased porosity (air permeability) of thesamples except at 0.2% NFC dosage. Air permeability as a function of NFCdosage is shown in FIG. 1G.

The primary effect of NFC was seen as increased strength properties andto some extent as lower porosity. Drainage time increased as NFC wasadded to furnish. Results were favorable for anionic NFC when comparedto native NFC.

EXAMPLE 4

The effect of nanofibrillated cellulose (NFC) with and without cationicstarch on selected paper properties and retention was tested at NFCdosage levels ranging from 0.1 to 1% (1 to 10 kg/t). All amounts aregiven on a dry-weight basis.

Raw Materials:

-   -   Chemical pulp: Kaukas Pinus produced by Kaukas pulp mill, beaten        to Canadian Standard Freeness (CSF) level of 605 mL.    -   Mechanical pulp: Pressure ground wood (PGW) from Kaukas paper        mill, refined to CSF level 71 mL.    -   Modified nanofibrillated cellulose (“Anionic NFC”): UPM        Biofibrils AS83, lot 11851, supplied as a gel with solids        content 2.52% by weight, available from UPM Kymmene Corp. in        Helsinki, Finland. The Anionic NFC was modified to result in a        surface charge that was more anionic than unmodified NFC.    -   Reference (“REF”): without nanofibrillated cellulose.    -   Cationic starch: RAISAMYL 70021, dry solids content 0.040%,        available from Chemigate, Finland.    -   Retention aid: cationic polyacrylamide (FENNOPOL 3400R).    -   Water (osmotically purified)    -   No filler was used

Method:

1. Dilution and activation of nanofibrillated cellulose:

-   -   A. Anionic NFC was diluted to 0.3% solids with water.    -   B. The diluted NFC composition was mixed with an immersion mixer        (BAMIX), carried out in three 10 s mixing periods.

2. Preparation of samples:

-   -   A. Mechanical pulp and chemical pulp were mixed at a ratio of        3:1 (mechanical pulp to chemical pulp).    -   B. Cationic starch was added to the samples according to TABLE        3.    -   C. Samples were mixed for 15 minutes after addition of starch.    -   D. Anionic NFC was added to the samples according to TABLE 3.    -   E. The samples were mixed for 5 minutes.    -   F. Retention aid was added to the samples at 50 g/t immediately        prior to sheet making.

3. Sheets were prepared from each sample using a circulation sheet mold.

4. Sheets were dried using gloss plates and air conditioned beforemeasurements.

TABLE 3 NFC Dosage. Sample NFC Dose (kg/t) Cationic Starch (kg/t) 1 0 02 1 0 3 4 0 4 7 0 5 10 0 6 0 10 7 1 10 8 4 10 9 7 10 10 10 10

The sheets were evaluated for various properties, including drainagetime, grammage (weight per area), bulking thickness, bulk density,tensile strength, stretch at break, tensile energy absorption (TEA), TEAindex, tensile stiffness, tensile stiffness index, breaking length,bonding strength (Scott Bond), air permeability (measured by theBendtsen method), and retention. Retention was determined by measuringweight of material going in to each sheet mold vs. the weight of sheet.Results are shown in TABLE 4 and FIGS. 2A-2G.

TABLE 4 Results. SAMPLE 1 2 3 4 5 6 7 8 9 10 Starch dose (kg/t) 0 0 0 00 10 10 10 10 10 NFC does (%) 0 0.1 0.4 0.7 1 0 0.1 0.4 0.7 1 Drainagetime (s) 14.5 13.8 14.4 14.6 14.5 12.1 11.8 12.8 12.7 12.9 Grammage(g/m²) 60.6 61.0 60.7 60.9 60.6 61.0 61.1 59.8 60.2 60.1 Bulkingthickness 116 117 117 115 114 112 114 114 114 114 (μm) Apparent bulk 523523 521 531 532 543 538 526 530 528 density (kg/m³) Tensile strength2.51 2.63 2.67 2.72 2.78 2.95 2.93 2.69 2.89 3.05 (kN/m) Tensile index41.5 43.1 44.0 44.6 45.9 48.3 47.9 45.0 48.0 50.7 (Nm/g) Stretch atbreak (%) 2.39 2.36 2.36 2.38 2.51 2.62 2.57 2.50 2.51 2.60 TEA (J/m²)42 43 44 45 49 53 51 46 50 54 TEA index (J/kg) 690 705 720 737 806 865835 771 827 905 Tensile stiffness 284 298 302 306 308 298 292 281 301313 (kN/m) Tensile stiffness 4.68 4.89 4.97 5.02 5.08 4.89 4.78 4.705.00 5.21 index (MNm/kg) Breaking length (m) 4229 4395 4487 4551 46854927 4883 4586 4899 5175 Bonding strength 308 310 311 327 344 449 443432 423 429 SB Low (J/m²) Air permeability 138 140 137 129 128 137 135150 145 144 Bendtsen (mL/min) Retention (%) 96.3 98.0 97.1 97.7 97.998.2 96.5 97.2 97.1 98.8

It was observed that adding cationic starch to the samples reduceddrainage time. The effect of NFC was visible with lowest dosage amount.Higher NFC amount either had not effect or increased drainage timeslightly. Drainage time as a function of NFC dosage is shown in FIG. 2A.

NFC addition increased retention above the reference at all dosagelevels when no starch was added. The reference (sample 1) had retention96.3%, and retention with NFC addition was 98.0% at dosage level 0.1%NFC (sample 2), 97.1% at dosage level 0.4% NFC (sample 3), 97.7% atdosage level 0.7% NFC (sample 4) and 97.9% at dosage level 1.0% NFC(sample 5). The retention increased about 0.8-1.7%-units compared withthe reference. In case of sheets containing starch, the reference withstarch (but no NFC) had high retention. The retention dropped at thelowest NFC addition, and then increased as a function of NFC, rising toabove the reference at 1% NFC addition. Retention as a function of NFCdosage is shown in FIG. 2B.

Sheets with cationic starch had slightly lower grammage than sheetswithout starch. However, no major differences were observed in grammage.Grammage as a function of NFC dosage is shown in FIG. 2C.

The differences in bulk density were very small in samples with andwithout starch. Bulk density as a function of NFC dosage is shown inFIG. 2D.

It was observed that NFC addition increased tensile above the referenceat all dosage levels for samples without starch. Addition of starchgenerally increased general the tensile level, but the changes intensile were not very systematic. Tensile index as a function of NFCdosage is shown in FIG. 2E.

NFC addition increased bonding (Scott Bond) above the reference at alldosage levels for samples without starch. Starch generally increased theScott Bond level. Bonding strength as a function of NFC dosage is shownin FIG. 2F.

NFC was found to reduce porosity for samples without starch. In case ofsheets containing starch, the effect of NFC was not very systematic. Airpermeability as a function of NFC dosage is shown in FIG. 2G.

It was concluded that the effect of Anionic NFC on the sheets withoutstarch were consistent. Improved retention, tensile strength, Scott bondand lowered porosity were obtained with Anionic NFC addition. Based onthe results obtained, Anionic NFC was found to have an effect on paperstrength properties and retention within the dosage range 0.1-1% insheets without cationic starch. The results for sheets with starch werenot as systematic and consistent as results without starch. At somedosage levels and for some parameters, the benefits were clear, whereasat some other dosage levels and for some other parameters, the benefitscould not be shown as clearly.

The method according to the invention is suitable in differentapplications to be used for manufacturing most different products.

The invention is not limited merely to the examples referred to above;instead, many variations are possible within the scope of the inventiveidea defined by the claims.

1. A method for producing a product in papermaking comprising: adding acomposition comprising anionically modified microfibrillated celluloseto a fiber suspension at a concentration of 0.1 to 10 wt-% anionicallymodified microfibrillated cellulose by weight of the fiber suspension toproduce a modified fiber suspension; and forming the product from themodified fiber suspension, wherein adding the anionically modifiedmicrofibrillated cellulose to the fiber suspension improves the strengthof the product.
 2. The method of claim 1, wherein anionically modifiedmicrofibrillated cellulose is prepared by modifying and fibrillatingcellulose or microfibril bundles comprising microfibrils.
 3. The methodof claim 1, wherein anionically modified microfibrillated cellulose isanionically modified nanofibrillated cellulose.
 4. The method of claim 1comprising adding 0.1 to 2 wt-% of the anionically modifiedmicrofibrillated cellulose to the fiber suspension.
 5. The method ofclaim 1 comprising adding about 1 wt-% of the anionically modifiedmicrofibrillated cellulose to the fiber suspension.
 6. The method ofclaim 1, wherein adding the anionically modified microfibrillatedcellulose to the fiber suspension improves retention of the product. 7.The method of claim 1, wherein the fiber suspension comprises fiberbased pulp formed by a chemical method, a fiber based pulp formed by amechanical method, or a combination thereof.
 9. The method of claim 1further comprising adding one or more fillers to the fiber suspension.10. The method of claim 9, wherein the one or more fillers comprises acationic filler added to the fiber suspension before adding theanionically modified microfibrillated cellulose.
 11. The method of claim1, wherein the composition further comprises a fiber-based solidmaterial.
 12. The method of claim 1, wherein the fiber suspensioncomprises fines.
 13. The method of claim 1 comprising adding a cationicpolyelectrolyte to the composition.
 14. The method of claim 1 comprisingadding about 0.1 to about 2 wt-% of a cationic starch to the fibersuspension.
 15. The method of claim 1 comprising adding an anionicpolyelectrolyte to the composition.
 16. The method of claim 1 comprisingadding inorganic nano- and/or microparticles to the composition.
 17. Themethod of claim 1, wherein adding the composition to the fibersuspension improves bonding strength SB of the product.
 18. The methodof claim 1, wherein adding the composition to the fiber suspensionimproves tensile strength of the product.
 19. The method of claim 1,wherein the product is paper.
 20. The method of claim 1, wherein theproduct is a product containing anionically modified microfibrillatedcellulose.
 21. A method for manufacturing a modified fiber suspension,the method comprising adding a composition containing anionicallymodified microfibrillated cellulose to a fiber suspension at aconcentration of 0.1 to 10 wt-% anionically modified microfibrillatedcellulose by weight of the fiber suspension to produce a modified fibersuspension, wherein adding the anionically modified microfibrillatedcellulose to the fiber suspension improves the strength of the product.